Discrete multi-tone (DMT) modulation is used in many types of data communication systems, among them Multi-carrier Very-high-speed Digital Subscriber Line (VDSL) modems, as well as Asymmetric DSL (ADSL). In these systems, N tones are modulated by QAM two-dimensional input frequency-domain symbols. A 2N-point Inverse Fast Fourier Transform (IFFT) then produces a corresponding time-domain symbol, expressed as a real baseband time-domain output signal of 2N real samples in each symbol period. At the receiving side, 2N samples are extracted from the time-domain signal during each symbol period. A FFT is used to demodulate the signal and recover the original QAM symbols on the N tones.
In order for this scheme to work, the timing of the receiver must be symbol-aligned with the transmitter, so that the N tones are mutually orthogonal. This means that for any given tone, only the energy transmitted on that tone will contribute to the received energy on that tone. The received signal, however, may still include interference due to echo and crosstalk. Ordinarily, these interfering signals would not be orthogonal to the tones of the received signal.
To mitigate this problem, VDSL provides a mechanism of digital duplexing, in which the transmissions in both directions on a DMT link are mutually synchronized so as to maintain orthogonality of the signal and echo. This feature, referred to as “MCM duplexing,” is described in Annex D of Technical Specification TS 101 270-2 V2.0.8 (2003-05), entitled Transmission and Multiplexing (TM); Access transmission systems on metallic access cables; Very high speed Digital Subscriber Line (VDSL); Part 2: Transceiver specification, promulgated by the European Telecommunications Standards Institute (ETSI, Sophia Antipolis, France, 2002), which is incorporated herein by reference. Digital duplexing techniques may also be used to synchronize neighboring links in a common “binder,” so as to orthogonalize crosstalk, as well. Further details of digital duplexing are described by Cioffi et al., in “G.vdsl: Digital Duplexing: VDSL Performance Improvement by Aversion of Frequency Guard Bands” (ITU Temporary Document NT-041, Nashville, Tenn., 1999), which is incorporated herein by reference.
In order to synchronize the transmitters at both ends of the VDSL DMT link, a cyclic prefix (CP) and cyclic suffix (CS) are added to each DMT symbol. The CP and CS together are referred to as the cyclic extension of the symbol, having a total length (in samples) CE. Thus, each symbol of length 2N is transmitted within a time-domain data block of length 2N+CE. The length of the cyclic extension is determined by the round-trip delay of the link and is typically much less than 2N. In addition, to reduce the effect of crosstalk among VDSL links in the same binder that are not mutually synchronized, the transmitters apply time windowing to the signals that they transmit.